diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/PlainMonthDayPrototype.cpp b/Userland/Libraries/LibJS/Runtime/Temporal/PlainMonthDayPrototype.cpp
index 9e5350ab3e1..591d11d6dcf 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/PlainMonthDayPrototype.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/PlainMonthDayPrototype.cpp
@@ -32,6 +32,7 @@ void PlainMonthDayPrototype::initialize(Realm& realm)
     define_direct_property(vm.well_known_symbol_to_string_tag(), PrimitiveString::create(vm, "Temporal.PlainMonthDay"_string), Attribute::Configurable);
 
     define_native_accessor(realm, vm.names.calendar, calendar_getter, {}, Attribute::Configurable);
+    define_native_accessor(realm, vm.names.calendarId, calendar_id_getter, {}, Attribute::Configurable);
     define_native_accessor(realm, vm.names.monthCode, month_code_getter, {}, Attribute::Configurable);
     define_native_accessor(realm, vm.names.day, day_getter, {}, Attribute::Configurable);
 
@@ -280,4 +281,16 @@ JS_DEFINE_NATIVE_FUNCTION(PlainMonthDayPrototype::get_iso_fields)
     return fields;
 }
 
+// 10.3.3 get Temporal.PlainMonthDay.prototype.calendarId
+JS_DEFINE_NATIVE_FUNCTION(PlainMonthDayPrototype::calendar_id_getter)
+{
+    // Step 1: Let monthDay be the this value
+    // Step 2: Perform ? RequireInternalSlot(monthDay, [[InitializedTemporalMonthDay]]).
+    auto month_day = TRY(typed_this_object(vm));
+
+    // Step 3: Return monthDay.[[Calendar]].
+    auto& calendar = static_cast<Calendar&>(month_day->calendar());
+    return PrimitiveString::create(vm, calendar.identifier());
+}
+
 }
diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/PlainMonthDayPrototype.h b/Userland/Libraries/LibJS/Runtime/Temporal/PlainMonthDayPrototype.h
index fdc7ea94127..f3a00aa2338 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/PlainMonthDayPrototype.h
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/PlainMonthDayPrototype.h
@@ -23,6 +23,7 @@ private:
     explicit PlainMonthDayPrototype(Realm&);
 
     JS_DECLARE_NATIVE_FUNCTION(calendar_getter);
+    JS_DECLARE_NATIVE_FUNCTION(calendar_id_getter);
     JS_DECLARE_NATIVE_FUNCTION(month_code_getter);
     JS_DECLARE_NATIVE_FUNCTION(day_getter);
     JS_DECLARE_NATIVE_FUNCTION(with);
diff --git a/Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainMonthDay/PlainMonthDay.prototype.calendarId.js b/Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainMonthDay/PlainMonthDay.prototype.calendarId.js
new file mode 100644
index 00000000000..7ac8fb01c27
--- /dev/null
+++ b/Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainMonthDay/PlainMonthDay.prototype.calendarId.js
@@ -0,0 +1,15 @@
+describe("correct behavior", () => {
+    test("calendarId basic functionality", () => {
+        const calendar = "iso8601";
+        const plainMonthDay = new Temporal.PlainMonthDay(5, 1, calendar);
+        expect(plainMonthDay.calendarId).toBe("iso8601");
+    });
+});
+
+describe("errors", () => {
+    test("this value must be a Temporal.PlainMonthDay object", () => {
+        expect(() => {
+            Reflect.get(Temporal.PlainMonthDay.prototype, "calendarId", "foo");
+        }).toThrowWithMessage(TypeError, "Not an object of type Temporal.PlainMonthDay");
+    });
+});